Chlorobromination of benzene in the presence of high energy radiation

ABSTRACT

Chlorobromination of benzene to produce principally 1-chloro2,3,4,5,6-pentabromocyclohexane is accomplished by a process comprising contacting chlorine, bromine and benzene in the presence of high energy radiation.

United States Patent David E. Harmer Midland, Mich.

Jan. 11, 1968 Oct. 26, 1971 The Dow Chemical Company Midland, Mich.

[ 72] inventor [2i 1 App]. No. [22] Filed [45] Patented [73] Assignee[54] CHLOROBROMINATION OF BENZENE IN THE PRESENCE OF HIGH ENERGYRADIATION [50] Field ofSearch ..204/l63 HE [5 6} References Cited OTHERREFERENCES Harmer et al., Nuclear Eng, Chem. Eng. Prog. SymposiumSeries, Vol. ll (1954) p. 253 TK 900LN7] Primary Examiner-Benjamin R.Padgett Attorneys-Griswold and Burdick and C. E. RehbergCIILOROBROMINATION OF BENZENE IN THE PRESENCE OF HIGH ENERGY RADIATIONBACKGROUND OF THE INVENTION Chlorobromocyclohexanes have been producedby contacting chlorine gas, bromine and benzene in the presence ofelectric light bulbs. See Emschwiller and Sacouney, Bull. Chim. Soc.,France, 16, 118 (1949). There are, however, problems encountered in thephotocatalyzed production of chlorobromocyclohexanes. One of the mostsignificant problems is the relatively shallow penetration of this typeof radiation in the dense reaction media employed. This problem isenhanced further by the addition product coating the inside of thereaction vessel. As a result, violent agitation is necessary to make thereaction proceed at a reasonable rate. The chlorobromocyclohexaneproducts of this reaction are difficult to purify, have small crystalsize which leads to handling difficulty and have lower melting pointsthan their purified counterparts.

SUMMARY OF THE INVENTION It has now been found that benzene ischlorobrominated to a product which is principallyl-chloro-2,3-4,5,6-pentabromocyclohexane by a process comprisingcontacting benzene, chlorine and bromine in the presence of high energyradiation. While not necessary to the invention, it is preferred tocontact the reactants in the presence of an inert solvent. By using thisprocess, it is no longer necessary to violently agitate the reactants,reaction rates are essentially constant throughout the reaction period,the product needs little, if any, purification, crystal size is largerand a higher melting point than was previously obtainable for the crudeproduct is achieved.

ln order to practice the invention, benzene, chlorine and bromine arecontacted in any convenient manner in the presence of high energyradiation. The product is then separated from the reaction productmixture by conventional methods. As was previously mentioned, the use ofan inert solvent ispreferred and said solvent can be precharged to thereaction vessel, added continuously during reaction, added incrementallyas the reaction proceeds, or added continuously or incrementally foronly a portion of the reaction time. Similarly, the benzene and thehalogen reactants independently can be added in the same manner as wasdescribed for the solvent addition. It is preferred to add the reactantseither incrementally or continuously throughout the reaction period orfor a portion of the reaction period. Of course, a recycle of unreactedbenzene, halogen reactants and solvent, or any one or two or more ofthem, can be employed so that either a batch or continuous process canbe utilized.

The term high energy radiation as used herein denotes that type ofradiation which will penetrate aluminum foil of 0.01 mm. thickness.Examples of high energy radiation suitable for this invention includeelectromagnetic energy (e.g. photons, gamma rays), high-velocityelectrons (beta rays), or other corpuscular beams such as neutrons,protons, alpha-particles, or deuterons such as those obtained fromnuclear reactors or accelerators like a Van de Graaff machine. Cobalt-60and cessium-l27 are particularly desirable sources of gamma rays.

The degree of high energy radiation necessary must be at leastsutficient to form sites for free radical addition. Of course, theradiation need not be administered throughout the reaction period oreven to all of the reaction vessel. The reaction time and radiationintensity are interrelated so that lower radiation intensities requirelonger reaction times than do the higher radiation intensities. Theduration of the period during which the reactants are subjected toradiation may suitably vary from a fraction of a second to many hoursdepending on the intensity of the radiation. It is preferred to use ahigh energy radiation source of electromagnetic energy with an energy ofbetween 0.1 and 10.0 mev. administered at radiation intensities varyingfrom i kilorad per hour to or megarads per hour (most preferably from 20kilorads per hour to l megarad per hour). Of course, a corpuscular beamof high energy radiation would have a much larger range of usefulradiation intensity. The beam energy can vary from 0.1 mev. to around 20mev. administered at radiation intensities between I kilorad per hour to2000 megarads per second (the latter would be an instantaneous dose rateadministered to an exposed portion of the total volume of the reactionsystem).

In order to produce l-chloro-2,3,4,5,6-pentabromocycloheane, the molarratio of bromine to chlorine can vary widely. Better yields are found atratios between 5:l and 1:3. Of course, bromine chloride can be used inthe chlorobromination of benzene. The molar ratio of bromine pluschlorine (or bromine chloride) to benzene can also vary widely, however,preferably it is between 6:] to lzl and most preferably between 3:] andlzl.

The solvent, when used, is any solvent which is liquid and substantiallyinert under the reaction conditions. It is preferred to use a solvent inwhich chlorine is soluble, e.g. carbon tetrachloride, methylenechloride, chloroform, trichloroethane, hexane or petroleum ether. Whenused, the solvent is preferably precharged to the reaction vessel.

The temperature range is not critical to this invention and can bevaried widely. Suitably, it is between 50 and C. and is preferablybetween l0 and 40 C. The preferred pressure is atmospheric butsubatmospheric and superatmospheric pressure can be employed.

The product is useful as a fire-retardant. It can be incorporated intoplastics to increase their resistance to flame and heat. The productproduced by this method, when used without further purification, iscomparable to the material produced in the prior art process which hasbeen specially purified.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Following are specific examplesof the practice of this invention.

The examples numbered 1, 2 and 3 were conducted in a two-liter kettletype batch reaction vessel constructed of glass and equipped with apaddle-type agitator, a condenser, a thermocouple well and inlet lines.Reaction temperature was controlled by means of a constant temperaturebath with secondary cooling fluid recirculating through the jacket ofthe reaction vessel. Radiation intensities were determined by ASTMMethod P-l67l-59. Proper allowances were made for decay of radioactivesource and calculated radiation attenuation within the reaction mixture.The method of addition of reagents to the reaction vessel was asfollows: The methylene chloride was precharged to the reaction vesselbefore the start of each batch. Bromine was added either in a batch atthe start of the reaction cycle, or incrementally in 25 ml. portionsthroughout the time of reaction. Benzene was either fed continuouslyduring the reaction or as a batch prior to the beginning of it. Chlorinegas and continuous benzene feed were metered into the reaction by meansof calibrated rotameters.

Upon completion of the reaction cycle, unreacted halogens in the mixturewere neutralized with 10 percent sodium hydroxide solution. Followingthis the aqueous layer was decanted and the product slurry in itssolvent was filtered on a fritted glass funnel. The filter cake thusattained was tray vacuum-dried at 70 C. for 4 hours. The product wasevaluated by determining 3 melting points: the onset of melting, thetemperature at which sufficient melting occurred to form a meniscuswithin the melting point tube, and the final melting point when allmaterial became liquified.

Cobalt-60 was used as a radiation source and had an average energy ofaround 1.25 mev. In each case the source was located adjacent to thereaction system.

As used herein, Br, efficiency is defined as the portion of totalbromine charged to the reaction vessel which is found in thechloropentabromocyclohexane product.

When longer reaction times are employed for carrying out thechlorobromination process, much lowered intensity of radiation can beused. Examples are provided by the data of table iI. The method ofprovided out the runs shown in table 11 is the same as that described inexample 1.

1238 ml. of till- C1 was precharged to the reaction vessel with 5 ml. ofHOl-l. 50 ml. of Br was precharged and thereafter 25 ml. was added every78 minutes for a total of 393 ml. C]: was added at a rate of 22 gramsper hour for the first hour and thereafter at a rate of 6.58 grams perhour. The benzene was added at a rate of 16.2 ml. per hour for 12 hoursstarting 1 hour after the start of the run. The reaction temperature was25 C. i 3 and the total run time was 32 hours. The results are reportedin table ll below.

EXAMPLE 1 TABLE 11 1224 Grams of bromine was either precharged or addedshotwise to the reaction vessel. Chlorine was fed in at a rate of 22.2grams per hour for the first hour and at a rate of 17.0 Run Number gramsper hour thereafter. Benzene was fed at a rate of 41.9 ml. per hour asindicated below. 1224 ml. of Ch Cl was 5 precharged to the reactionvessel. The total run time was 13 hours, the reaction temperature was 25C i 3 and the radiation intensity was 126.5 kilorads per hour. Resultsare shown in table 1 below. Grams product 993 820.7 I 31 Hr, efficiency63.1 52.5

Melting Points "C:

Initial 191 193 Meniscus 19s 196 Run Number Final I99 I99 Bromine feedShotwise 25 ml. precharge precharge EXAMPLE 3 at 30 min.

intervals Hume feed hm hm I2 pmhmge Using the same procedure andequipment as described In hm examples 1 and 2, 1224 grams of Br, wascharged to the ves- Gfamvwdw 828-7 607-6 194.8 sel. The benzene feedrate was 41.9 ml. per hour and the z'if fi C 53 5 chlorine feed rate was22.2 grams C1, per hour for the first e "I DID I y 192 19 4 197 hour and17.0 grams Cl per hour thereafter. The reaction Meniscus 9 193 202temperature was C. I 3 and the radiation intensity was Final I98 201 204126.5 kilorads per hour. 1224 M1. of CH,Cl, was precharged and theresults are re orted below in table Ill. P

TABLE 111 Total Chlo- Percent run rine yield Melting points, C. Runtime, feed, Grams base Number hours hour Bromine Iced Benzene feedproduct on Br- Init Men Fin. 10 10 Precharge Last 9 huurs 548 193 196199 10 10 o. ...do s 32 104 m7 200 10 10 Shotwlse 25 ml. at .do. s44 411210 19s 19s 30 min. int. 13 t1 hourslhour 605 39 191 1114 198 afterstart to 10 hours. 10... 13 13 ..do. Last 12 hours 860 192 194 197 11 1313 .do fihours l hour 727 46 192 196 after start to 10 hours. is 13....do Last 12 hours 828.7 53 192 1116 19s 13 13 Precharge. o 607.6 38.7194 198 201 13 d Precharge 194.8 12.4 197 202 204 1 First 10 hours.

EXAMPLE 2 EXAMPLE 4 In this example a modification of the reactionsystem was made to allowthe radiation exposure to take place in a sidearm portion. The reactant mixture was continuously pumped through thisirradiated side arm and back again into a stirred holding vessel.

The holding vessel for this system consisted of a glass container ofapproximately 2 liters capacity. lt was continuously agitated during therun by means of a paddle-type stirrer. The reaction mixture was takenfrom the bottom of the vessel and recirculated by means of apolyethylene or glass centrifugal pump, the discharge of which wasconnected to a small side arm of approximately 225 ml. capacity. Thisside arm was ir' radiated by placing a group of 5 rods or cesium-137 ina circle around it. The pump and holding tank were not irradiated sincea wall of lead shielding blocks was positioned between the side arm andthe remainder of the system. Temperature control was achieved by meansof a jacket through which cooling water flowed, surrounding the side armreactor section. The vent for the system was connected to a scrubber toremove corrosive gases. Removal of condensable liquids and chlorine fromthe system was minimized by passing the offgases through a condenseroperating under refrigeration, prior to their entering the scrubbersystem.

Dosirnetry for this arrangement was carried out using a separate glassvessel located within the same configuration of cesium source rods. Thestandard Fricke dosimeter system was employed (ASTM-167 1-59).

At the beginning of the -hour run, the precharge of methylene chlorideand bromine was added to the system. Chlorine was then fed to the systemfor a period of 1 hour in the presence of the radiation, but withoutbenzene feed. The remaining reactants were then fed continuously duringthe next 14 hour period, after which time the run was terminated.Benzene was fed by means of a dual syringe pump, using a calibrateddropping funnel as an additional check on quantity fed to the reaction.

Chlorine was measured through a calibrated rotameter. Bromine was placedin a dropping funnel attached to the reactor prior to each run. It wasadded remotely in increments each hour throughout the run. This was a 15hour run at a radiation intensity of 1.13 megarads per hour administeredto the side arm of the reactor. The reaction temperature was 25 C. 3 and7 ml. of H011 was added as a precharge The results of these runs arepresented in table IV.

TABLE IV Run Number l5 l6 17 I8 Precharge:

CI-I,CI,,ml. I875 I875 I875 1875 Br,, gram: 277.6 202.7 355.6 355.6Total Br, feed grams 1406.3 1556.3 1875 1875 Total benzene feed 506.3506.3 675 675 Total Cl, feed grams 180 180 241.9 357.3 Grams product1033.6 1073.1 1076.4 1485.5 Yield based on Br, 53.6% 44.5% 61.4%

Melting Points, C: Initial 190 192 193 190 Meniscus 193 197 196 192 IFinal 198 200 200 196 EXAMPLE 5 20 sufficient to form sites for freeradical addition.

2. A process as defined in claim I wherein the reactants are contactedin the presence of a substantially inert solvent.

3. A process as defined in claim 1 wherein the bromine to chlorine molarratio is between 5:1 and 1:3.

4. A process as defined in claim 1 wherein the molar ratio of bromineplus chlorine to benzine is between 6:1 and 1:1.

5. A process as defined in claim 1 wherein the bromine is addedincrementally or continuously throughout the reaction period or aportion thereof.

6. A process as defined in claim 1 wherein the benzene, chlorine andbromine are independently added incrementally or continuously throughoutthe reaction period or a portion thereof.

7. A process as defined in claim 1 wherein the pressure is atmospheric.

8. A process as defined in claim 1 wherein the high energy radiation hasan energy between 0.1 and 10.0 mev. administered at a radiationintensity between 1 kilorad and 20 megarads per hour.

9. A process as defined in claim I wherein the temperature is between 50and C.

10. A process as defined in claim 8 wherein the intensity is between 20kilorads and 1 megarad per hour and the type of energy is gammaradiation.

11. A process as defined in claim 2 wherein the inert solvent isprecharged to the reaction vessel and is methylene chloride.

# i i l

2. A process as defined in claim 1 wherein the reactants are contactedin the presence of a substantially inert solvent.
 3. A process asdefined in claim 1 wherein the bromine to chlorine molar ratio isbetween 5:1 and 1:3.
 4. A process as defined in claim 1 wherein themolar ratio of bromine plus chlorine to benzine is between 6:1 and 1:1.5. A process as defined in claim 1 wherein the bromine is addedincrementally or continuously throughout the reaction period or aportion thereof.
 6. A process as defined in claim 1 wherein the benzene,chlorine and bromine are independently added incrementally orcontinuously throughout the reaction period or a portion thereof.
 7. Aprocess as defined in claim 1 wherein the pressure is atmospheric.
 8. Aprocess as defined in claim 1 wherein the high energy radiation has anenergy between 0.1 and 10.0 mev. administered at a radiation intensitybetween 1 kilorad and 20 megarads per hour.
 9. A process as defined inclaim 1 wherein the temperature is between -50* and 100* C.
 10. Aprocess as defined in claim 8 wherein the intensity is between 20kilorads and 1 megarad per hour and the type of energy is gammaradiation.
 11. A process as defined in claim 2 wherein the inert solventis precharged to the reaction vessel and is methylene chloride.